In ITU-T Recommendations G.8031/Y.1342, it is defined that the APS (Automatic Protection Switching) protocol and linear protection switching mechanisms for point-to-point VLAN (Virtual LAN) based ETH (Ethernet layer network) SNC (Sub-Network Connection) in Ethernet transport networks.
The linear protection switching mechanisms to be applied to VLAN-based Ethernet networks as described in ITU-T Recommendations G.8010/Y.1306. Protection switching is a fully allocated survivability mechanism. It is fully allocated in the sense that the route and bandwidth of the protection entity is reserved for a selected working entity. It provides a fast and simple survivability mechanism. It is easier for the network operator to grasp the status of the network (e.g., active network topology) with a protection switching than with other survivability mechanisms such as RSTP (Rapid Spanning Tree Protocol).
Two kinds of protection switching architecture are specified, they are linear 1+1 protection switching architecture and linear 1:1 protection switching architecture. The linear 1+1 protection switching architecture operates with either unidirectional or bidirectional switching. The linear 1:1 protection switching architecture operates with bidirectional switching.
In the linear 1+1 protection switching architecture, a protection transport entity is dedicated to each working transport entity. The normal traffic is copied and fed to both working and protection transport entities with a permanent bridge at the source of the protected domain. The traffic on working and protection transport entities is transmitted simultaneously to the sink of the protected domain, where a selection between the working and protection transport entities is made based on some predetermined criteria, such as server defect indication.
Although selection is made only at the sink of the protected domain in linear 1+1 protection switching architecture, bidirectional linear 1+1 protection switching needs APS coordination protocol so that selectors for both direction selects the same entity. On the other hand, unidirectional linear 1+1 protection switching does not need APS coordination protocol.
In the linear 1:1 protection switching architecture, the protection transport entity is dedicated to the working transport entity. However, the normal traffic is transported either on the working transport entity or on the protection transport entity using a selector bridge at the source of the protected domain. The selector at the sink of the protected domain selects the entity which carries the normal traffic. Since source and sink need to be coordinated to ensure that the selector bridge at the source and the selector at the sink select the same entity, APS coordination protocol is necessary.
Two kinds of operations, revertive and non-revertive operations, are also specified.
In revertive operation, normal traffic signal is restored to the working transport entity after the condition(s) causing a switch has cleared. In the case of clearing a command (e.g., a Forced Switch command), this happens immediately. In the case of clearing of a defect, this generally happens after the expiry of a “Wait-to-Restore” timer, which is used to avoid chattering of selectors in the case of intermittent defects.
In non-revertive operation, normal traffic signal is allowed to remain on the protection transport entity even after a switch reason has cleared. This is generally accomplished by replacing the previous switch request with a Do not Revert request, which is low priority.
In addition, several states are specified in G.8031 and the priority between them is also specified.